The present invention relates to a print engine that includes error recovery, and in particular to a print engine with multiple language capability that includes error recovery.
Personal computers have become commonplace on the desks of most office workers. Typically, much of the work product of such computers is intended to be transformed into hard copy using a printer having digital imaging technology. A typical printer configuration for this purpose is a printer dedicated to each personal computer. However, dedicated printers are typically inexpensive laser printers which have limited functions and features, such as a limited tray capacity and no finishing capability. More importantly, inexpensive laser printers typically handle only one page description language.
Expensive high speed laser printers normally have flexible finishing and copy sheet capability which allows for example, custom printing and finishing of work product. Such a laser printer would be highly desirable for each personal computer, but is cost prohibitive. In practice, personal computers are frequently networked together which permits a single printer to be used simultaneously by a plurality of different clients, such as personal computers or the like.
Referring to FIG. 1, data processing terminals 10a, 10b and 10c, such as personal computers, transmit electronic print documents through a local area network (LAN) 12 to a selected printer 14a or 14b for printing. In particular, the LAN 12 provides a medium by which different terminals are able to share resources such as printers 14a, 14b, file servers, a printer server(s) 16, and scanners. Integration of shared resources has been addressed by LAN managers using different network protocols, such as Ethernet and Token Ring, to make different devices running different network-protocols transparent to the terminals. Most printer servers 16 and many printers 14a, 14b support a variety of print drivers using different page description languages (PDLs).
A PDL is a method of describing a printed page(s) in a printer independent format. No single standard PDL presently exists, and as a result a number of industry standards have emerged. A PDL establishes an interface between a print driver or client (terminal or personal computer), and a printer server or printer. Several currently existing PDLs include PostScript (PS), Hewlett Packard Printer Control Language (HP-PCL), and Interpress Page Description Language.
In existing systems, the processing of the PDL data takes place using an interpreter, which reads the received PDL data (typically text) and creates corresponding bit mapped data which is provided to the printer engine. The printer engine transforms received digital data into areas of black or colored ink on the paper or other medium fed into the printer.
In networked systems, in which several users at various terminals are sharing a single printer, a desirable feature is that each terminal (i.e., each user) will “see” the network accessible printer as “dedicated” to that user. In other words, when a user transmits a document to be printed on the central printer, the user desires the same service as if the printer was dedicated to that terminal. In fact, most printer interfaces used on personal computers are designed to operate in a fashion that appears to the user as a one-to-one relationship between the terminal and the printer. A user does not want to observe any delays caused by an undesirable interaction of his print job with another job from a different user to be printed on the central networked printer at generally the same time. Further, an important customer requirement is that the networked printer is capable of receiving document data in a variety of different PDLs. As mentioned above, two common types of PDLs are PostScript and HP-PCL.
Many printers (or print servers) incorporate a plurality of language interpreters to facilitate printing documents that are transmitted to the printer in a variety of PDL formats. In such a printing system, one of several PDL interpreters within the printer (or print server) is automatically selected depending upon the PDL format of the print data from a particular data processing device. Accordingly, the appropriate language interpreter must be selected for each print job. There are numerous patents describing printer controllers which include multiple interpreters, such as, U.S. Pat. No. 4,651,278; U.S. Pat. No. 4,965,771; U.S. Pat. No. 4,968,159; U.S. Pat. No. 5,036,476; U.S. Pat. No. 5,165,014; U.S. Pat. No. 5,226,112; U.S. Pat. No. 5,228,118; U.S. Pat. No. 5,303,336; U.S. Pat. No. 5,402,527; U.S. Pat. No. 5,475,801; and U.S. Pat. No. 5,592,683, all of which are incorporated herein by reference.
In an ideal situation the transmission and processing of printer data is error-free. As such, all of the print jobs will likely print properly. Unfortunately, errors periodically occur in the data transmitted from the terminal to the printer, data is lost or corrupted within the printer itself, protocol failures mix different data streams at the printer server or in the printer, and errors originate within the client or printer software. For example, if PostScript data is being transmitted to the printer and an error occurs, then the printer may interpret the data as standard text which likely results in printing an extensive number of pages containing “garbled” data.
Printer description languages typically include synchronization and setup data at the beginning, and at intermediate locations, within each document. When errors occur within a document, either a portion of the page will be corrupted while the remainder of the document prints properly, or the printer will become sufficiently confused and continue printing at the next synchronization point. Unfortunately, the next synchronization point for many documents is not until the next document, which results in the loss of the remainder of the current document. This technique of error recovery is typically used in printers that support only one PDL.
For printers that support multiple language interpreters it has been observed that frequently when an error occurs in a first document, the remainder of the document is lost in a manner similar to printers with a single language interpreter. Unfortunately, the next document is frequently likewise lost resulting in additional frustration to users.
Alternatively, the printer may request the retransmission of all or part of the PDL stream in which an error is found. However, retransmission requires a two-way communication between the printer and the originating producer of the PDL data which is typically not available in heterogeneous distributed networking environments.
What is desired, therefore, is an error recovery system for a print engine that does not result in the loss of the next document following an error.